1. Field of the Invention
The present invention relates generally to compact disc manufacturing, and more particularly, to methods and apparatus for temperature controlled spin-coating systems.
2. Description of the Related Art
Compact disc recordable (CDR) technology has received increased popularity due to increased consumer demands for storage devices having large storage capacities and fast reading and writing capabilities. Generally, CDR discs are similar to well known read-only compact discs (CDs) used in the music recording industry and computer software industry. However, fabricating CDR discs requires various processing steps that are dissimilar to those employed by CD manufactures. Initially, CDR disc manufactures use well known molding machines that are configured to receive a polycarbonate (i.e., plastic) material from one end and output a warm clear plastic disc at the other end. Once output from the molding machine, the discs are cooled and inspected for defects or abnormalities which may have been introduced during the molding process. If the polycarbonate disc meets quality control tolerances, the disc is moved to a chemical application station where a solvent based chemical is coated over the clear polycarbonate disc. Suitable well known solvent based chemicals may be obtained from CMR Technology, Inc., Trumble, Conn.
In "record once" recordable CDR technology, the solvent based chemical coated over the surface of the polycarbonate disc is a critical layer that typically defines the resulting performance ratings of recording and reading operations. The solvent based chemical generally acts as a programmable layer that changes characteristics when a recording laser is directed at the underside of the polycarbonate disc. Accordingly, during a subsequent reading operation, a reading laser is scanned across the disc and a sensor is able to distinguish recorded signals from non-recorded regions by examining the reflected light from the reading laser.
Nonuniformities in the chemical coating affect the reaction of the chemical coating to the reading laser. Therefore, the solvent based chemical applied over the polycarbonate disc must be extremely uniform in order to produce CDR discs that are sufficiently fast at both recording and subsequent reading operations. If the solvent based chemical is applied in a non-uniform manner over the surface of the polycarbonate disc, the recording and reading responses will suffer and therefore produce a slower recording and reading CDR disc.
As is well known in the art, CDR discs are classified and marketed as either 2X, 4X, 6X, 8X, etc., depending the CDR disc's recording speed. Although manufacturing costs associated with fabricating CDR discs having 2X, 4X, 6X, or 8X speeds are substantially equal, typical market prices for 2X discs are substantially lower than that of 8X discs. However, generating uniform chemical coatings over the surface of a polycarbonate disc has been found to be extremely challenging due to a number of factors, including increased temperatures generated in conventional spin-coating systems. By way of example, most commonly used spin-coating systems have chucks for holding discs in place while rotating the chuck at high revolutions per minute (RPMs). Of course, in order to rotate the chuck and disc at speeds suitable for spin-coating solvent based chemicals used in CDR technology, the mechanical friction and motors to rotate the chuck generate increased temperatures.
As CDR discs are spin coated in a manufacturing line where hundreds or thousands of discs are spin-coated one after another, the temperatures of the spin-coating systems may increase to temperatures reaching about 35 degrees .degree.C. Typically, when discs are placed over a chuck for spinning, the disc is only in contact with the chuck near the center radius where no information is written. The polycarbonate material which makes up the disc is a relatively good thermal insulator. Therefore, when a disc is placed in contact with the warm chuck, the disc will generally be warmer near the center radius and cooler near the edges. As a result, the solvent based chemical will be less viscous (i.e., thinner) near the center and more viscous (i.e., thicker) near the outer radius. The solvent also tends to be more viscous at the edge as a result of evaporation. Consequently, undesirable nonuniformities are introduced which degrade the quality of reading and writing operations.
FIG. 1 is a cross-sectional view of a CDR disc having non-uniformities believed to be caused in part by elevated temperatures in conventional spin coating systems. In this example, a disc 32 is shown having various chemical layers applied over the disc's surface. As described above, a solvent based chemical 36 is initially applied near an inner radius of the disc 32 in order to spread the chemical over the top surface of the disc once the disc is rotated to equilibrium speeds. As shown, the applied solvent based chemical 36 has a wavy top surface having a thinner profile near the radius and a thicker profile near the outer radius.
As illustrated, when the center of the disc 32 has a higher temperature, the solvent based chemical 36 is will be less viscous and therefore exhibit a thinner profile 36b. Conversely, the outer radius of the disc 32 will have a lower temperature because it is not in contact with the chuck. As a result, the solvent based chemical will be more viscous and have a thicker profile 36a.
Once the solvent based chemical 36 is applied over disc 32, disc 32 is removed from the spin coating system and placed into a sputtering machine where a gold material 38 is sputtered over solvent based chemical 36. Although the gold material 38 may have a more uniform profile throughout the disc surface, the non-uniformities of the underlying solvent based chemical 36 unfortunately mirror up. Once the gold material is sputtered on, the disc 16 is moved to another station where a suitable protective lacquer coating 40 is applied over the gold layer 38 and solvent based chemical 36. As is well known in the art, protective layer coating 40 serves to seal the various layers from ambient conditions and prolongs a CDR's useful life.
Once the lacquer coating 40 is applied, the disc 16 is placed into an ultraviolet curing station where lacquer coating 40 is appropriately hardened. At this point, the core CDR fabrication steps are complete and the CDR disc may then be recorded with suitable information using a recording head 46 that applies a recording layer beam 48 to an under surface of the CDR disc. During recording, the recording laser beam 48 reacts with solvent based chemical 36 which changes the chemical properties of solvent based chemical 36. As a result, the changed chemical properties produce a recorded spot 35 which identifies a recorded event. Of course, it should be appreciated that the non-uniform wavy characteristics of solvent based chemical 36 may detrimentally affect recording an reading speeds which consequently impact a CDR disc's value.
In view of the foregoing, there is a need for methods and apparatuses for controlling the viscosity of the chemical during spin-coating process to reduce non-nonuniformities in coatings which have changing viscosity as a result of temperature variation and evaporation.